This application relates to finishing the surface of metallic prostheses that are configured for implantation and deployment in a body cavity. Specifically, the application is directed to electropolishing small tubular metal prostheses such as stents and other tubular implants.
Currently, electropolishing is the state of the art method for finishing the surfaces of metallic implants and other metallic medical devices. The goal of electropolishing a metallic implant is to achieve a smooth, corrosion-resistant, biocompatible surface. From a manufacturing standpoint, the difficulty lies in achieving this pristine surface consistently throughout every surface of the device, and in maintaining that consistency from one device to the next.
The process of electropolishing is achieved by applying a voltage, or potential difference, between a device to be polished (such as a stent) acting as an anode, and a cathode while both the anode and the cathode are submerged in a conductive electrolyte bath. This arrangement permits current to flow from the stent as anode, through the electrolyte, to the cathode. When the parameters are adjusted properly, this process removes metal ions from the surface of the stent in such a manner as to smooth the surface to a mirror finish.
Although there are many critical parameters that determine the effectiveness of an electropolishing process, one especially important factor is the type of fixing structure (sometimes referred to as “fixture”) used to mount the stent on an anode to provide current (or potential difference) from the power supply to the stent and thence into the electrolyte solution. Typically, a tubular cylindrical stent is stretched over a cylindrical anode to provide enough contact to ensure a robust electrical connection between stent and anode, by which the stent itself can act as the anode during electrolysis. However, this method does not consistently electropolish the entire stent, because regions of the stent closer to the point of electrical contact with the anode will exhibit higher current density, and will be preferentially polished. This preferential polishing effect necessitates a multi-step polishing process that requires the stent and anode to be removed from the electrolyte, after which the stent is removed from the anode and then manually rotated or otherwise repositioned relative to the anode before being reinserted into the electrolyte so that polishing can continue. In general, the more electropolishing steps of this kind that are taken, the more consistent the process and the more even the resulting polish imparted to the stent. Depending on the size and design of the stent, this could require anywhere from five to ten steps or even more in some cases.
Thus there is a need in the art for a system and method to overcome the above described shortcomings in the art, by which the disadvantageous effects of preferential electropolishing may be reduced. The present invention addresses these and other needs.
There is yet another problem encountered in the art of electropolishing small metallic implants or stents, which arises under the following circumstances: In the known art, a stent may be suspended or supported in electrolytic solution for polishing while being connected to an electrically charged anode. Frequently, in this process, a cathode is positioned to surround the stent acting as anode as is shown in FIG. 5a. In this figure, a cylindrical shaped mesh 400 acting as cathode is positioned to surround the stent 444. The stent 444 is positioned on a holder 402 that includes an elongate cylindrical portion 403 (or, blade portion) which is given a positive charge via potential difference means 410. The blade portion attaches to a connector portion 405 that is the closest point on the holder 402 that connects to the power source or potential difference means. This arrangement provides an electric current into the electrolytic solution 404 that flows between stent 444 and cathode 400. For example, a platinum-iridium woven wire mesh may be used, that is rolled into a cylinder, and is placed in the electrolyte solution to surround the stent and function as a cathode 400. In the prior art, the diameter of the cylindrical cathode 400 is constant. In use of this prior art method, an electric current is passed through the stent 444, as anode, for a short duration. Thereafter the stent is removed from the electrolyte and manually rotated about its own axis, by for example 60 degrees before being reinstalled on the blade portion 403, so that the area that previously touched the blade portion will be exposed to the solution to ensure even polishing. The rotation step is typically performed three times. Thereafter the entire stent 444 is removed from the blade portion 403 and turned end-over-end before being placed back onto the blade portion for three more polishes, alignments and rotations.
This end over end rotation is used because the top of the stent becomes polished to a greater extent than the bottom of the stent while suspended in the polishing electrolyte mixture. It is believed that this effect is attributable to the fact that the top of the stent is closer to the point at which the holder 402 attaches to the connector portion 405—which is in direct contact with the electric power supply. Thus, the current flow density into the electrolyte is greatest adjacent the point of connection of the stent to the power supply 410, and becomes attenuated toward the bottom portion of the stent where it is furthest from the point of contact with the power supply 410. The attenuated current flow density leads to a lower rate of polishing, and a resulting uneven final result.
Thus, there is a further need in the art for a system and method to overcome the above described shortcomings in the art. The present invention also addresses these and other needs.